16 mm sound troubleshooting (no sound)

[Andrew Lepore]

Hello, I have a Bell & Howell 16 mm projector. The sound did work but no longer works. I suspect the sound card is likely the issue, perhaps failed components. I'm a mechanical engineer with a decent level of knowledge in electronics, but poor at circuit board diagnosis. I'd like to learn the basics on plan of attack on solving circuit board problems. Perhaps my issues are in old/bad capacitors or other components. Attached is a pic of the board and projector schematic. Any advice/help either in circuit or out of circuit tests and a path to root cause would be great.
Thanks for any help. Andy

 

[Alec_t]

Welcome to EP.
I'd suggest the first steps would be to
1) check that the voltages at the secondaries of the mains transformer are reasonably correct (taking care to keep well clear of the LIVE primary side of the transformer),
2) identify the various power rails on the pcb and see if their voltages are correct.

 

[Andrew Lepore]

Roger that. Thank you.

 

[shrtrnd]

Alec_t's advice is the best start.
Typically, what happens is the aluminum electrolytic capacitors age and leak. Sometimes it's just the caps, but
often when they get too far out of spec they can take-out some of your semiconductors.
Finding out what your voltages are is a great way to try to isolate the problem.
Good luck in finding a quick fix.

 

[Andrew Lepore]

Shrtrnd,
I will check the voltages in addition to checking the testing the Capacitors.
Thanks for the advice.

 

[Audioguru]

The schematic was upside down, backwards and dark grey. I fixed it but the numbers are hard to read.

 

[Andrew Lepore]

Hi, are there any numbers in particular you would like me to see if I can read off the print? Perhaps if circled on the image uploaded. I got the schematic off the web for a Bell & Howell 1535 projector.
Thank you.

 

[Audioguru]

It looks like many DC voltages are shown on the schematic. Did you measure your amplifier and see what is wrong?

 

[CDRIVE]

This is my second try to paste the image. I hope it's successful.
Chris

Edit: It's too large.